1. Technical Field
The present disclosure relates to a camera calibration method, a recording medium, and a camera calibration apparatus.
2. Description of the Related Art
Calculation of camera parameters, in other words calibration of a camera requires a function which evaluates the camera parameters based on a two-dimensional coordinate pair on a two-dimensional image or a three-dimensional coordinate set in a three-dimensional space. The two-dimensional coordinate pair on the two-dimensional image is also referred to as an image coordinate pair, and the three-dimensional coordinate set in the three-dimensional space is also referred to as a world coordinate set. The evaluation function described above expresses the difference between the calculated camera parameters and the correct camera parameters, and if the calculated camera parameters coincide with the correct camera parameters, the evaluation function is 0. Camera calibration employing a conventional evaluation function may include: first preparing a pair of a world coordinate set A1i of a target point A0i in a three-dimensional space and an image coordinate pair A2i of a point corresponding to the target point A0i by use of a calibration apparatus; then obtaining an image coordinate pair A3i of a point which is a projection of the world coordinate set A1i of the target point A0i onto an image based on the camera parameters; and using as an evaluation value a total sum ΣA4i (also referred to as a reprojection error) of distances A4i between the image coordinate pair A2i and the image coordinate pair A3i. Another case may include using as an evaluation value the difference between the length measured for the target point with a stereo camera and the length measured in advance from the stereo camera to the target point. For example, the former technique, which uses the pair of a world coordinate set and an image coordinate pair, is disclosed in Roger Y. Tsai, “A Versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off-the-Shelf TV Cameras and Lenses”, Journal of Robotics and Automation, IEEE, August 1987, Vol. RA-3, No. 4, pp. 323-344 (Non-Patent Document 1) and Zhengyou Zhang, “A Flexible New Technique for Camera Calibration”, IEEE Transactions on Pattern Analysis and Machine Intelligence, IEEE, 2000, Vol. 22 (11), pp. 1330-1334 (Non-Patent Document 2). The latter technique, which uses a stereo camera, is disclosed in Japanese Unexamined Patent Application Publication No. 2012-202694 (Patent Document 1).